class MaoPao(object):
    def add(str):
        sum = list(str)
        '''sum2 = []
        for i in sum:
            if i not in 10:
                sum.remove(i)
            else:
                return sum2.append(i)'''
        for i in range(0,len(sum)-1):
            for j in range(i,len(sum)-i-1):
                if sum[j] > sum[j+1]:
                    sum[j],sum[j+1] = sum[j+1],sum[j]
        return sum